The present invention relates to a light scanning unit used in a laser printer or a digital copier, for example, and more particularly a light scanning unit of the over-illumination type in which the width of the main scanning direction of a luminous flux incident on a polygon mirror (the direction along the polygon mirror rotating direction) is wider than the width of the reflecting surface in the polygon mirror in the main scanning direction.
As a light scanning unit of the over-illumination type by the inventor of this application, there is the one discloses by Jpn. Pat. Appln. KOKAI Publication No. 2002-328323=U.S. patent application Ser. No. 10/131,207 (Filed 25 Apr. 2002).
In the above-mentioned over-illumination type light scanning unit, the width of the main scanning direction of a light beam deflected by a polygon mirror changes depending on the scanning angle (position angle), as explained in the above-mentioned previous application.
However, even in the previous application by the inventor of the present application, it is not completely solved that when the light beam incident on the polygon mirror forms an angle on the main scanning plane with respect to the optical axis of the image forming optics (when the light beam enters obliquely), the variation in the beam section diameter becomes asymmetrical with respect to the center of the optical axis of the image forming optics.
Further, it is also as proposed by the inventor of the present application that when a light beam is applied to an optional reflecting surface or a polygon mirror from the front (close to the front) of the main scanning direction in order to decrease the variations in the beam section diameter on the image surface (photodetector), and the light beam reciprocating between the optional reflecting surface of the polygon mirror and a group of lenses provided between the polygon mirror and the photodetector is applied to the image area of a photoconductor to form the image, the degradation of the image quality can be suppressed by a certain degree by providing antireflection coating on the surface of an optional lens. However, there is a problem of deterioration in the lens surface precision and optical characteristics, in addition to an increase in the lens cost.